Integration of Reservoir Engineering and Geomechanics to Ensure Horizontal Well Drilling Success in a Complex Waterflooding Development Fault Block – A Case Study

Horizontal well drilling is more challenging during the redevelopment of mature fields because the reservoir pressure is not original due to production and injection. The geomechanical model used for drilling must consider such effects. This paper demonstrates the integration of reservoir engineering (RE) and geomechanics (GM) studies to ensure horizontal well drilling success with reduced formation damage in a complex faulted, waterflooding development block. This is a case study of an offshore development field in the South China Sea. The overall approach is to make an integrated geomechanical study and reservoir engineering analysis, using a 1D GM model, dynamic model simulation, and comprehensive geological analysis. The 1D geomechanics model for the offset wells was built using geological data, drilling data, engineering data, wireline/real-time logging data and laboratory rock mechanical testing data. The calibrated GM model has the capability to accurately model the pore pressure, collapse pressure and fracture pressure, and find out the root cause of wellbore instabilities. The reservoir simulation model will provide the time dependent pore pressure and the uncertainties for the GM model. Thus, the integrated model between two disciplines will deliver a robust forecast. In the faulted block, the reservoir pressure is unevenly distributed with some wells overpressured due to waterflooding and some wells still showing depletion due to poor sandstone connectivity. A horizontal study well in the oilfield encountered serious drilling problems such as many tight hole events, reaming requirements and the need to sidetrack twice. The result was the well was completed with a lateral much shorter than planned. A neighbouring horizontal well was abandoned due to wellbore losses. A comprehensive GM model was developed in which the pore pressure comes from the reservoir dynamic simulation output at the time of well drilling. The calibrated GM model indicates that the in situ horizontal stresses are affected by pore pressure changes as are the collapse pressure and fracture gradient. The comprehensive model can explain the reason why highly deviated or horizontal trajectories in one shaly section of the study block may easily suffer wellbore collapse. Modelling can also propose the optimal well trajectory and safe mud weight window for future drilling. The model is also used to analyze the fault leaking pressure and can explain the heavy mud losses that occurred in one well that penetrated a major fault. The workflow has guided the success of several newly drilled horizontal wells in the study block. It may be a hard task to set the appropriate mud weight window to ensure wellbore stability and limit formation damage for horizontal wells during the redevelopment of a mature field. The integration between reservoir engineering and GM study can overcome such challenges. The best practices and workflow in this case study can serve as a reference for future situations.